It has long been controversial whether hematopoiesis progresses through ordered stages of determination as in embryonic development. This is due to the absence of a methodology capable of exactly determining the developmental potential of hematopoietic stem/progenitor cells. The multilineage progenitor (MLP) assay enabled us to discriminate among seven types of hematopoietic progenitors, which are multipotent progenitor p-MTB (capable of generating myeloid, T and B cells), bipotent progenitors p-MT, p-MB and p-TB, and unipotent progenitors p-M, p-T and p-B. Among these seven types, the p-TB type progenitor was found to be absent. These findings indicate that the process of lineage commitment proceeds through an ordered but not random process. By extending the area of investigation to include the erythroid lineage, more convincing evidence for the ordered process was obtained. Detailed and exact illustration of the process of hematopoiesis will provide an opportunity to revive hematopoiesis as one of the most fascinating targets of research in developmental biology.
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